Tetrahydropyrans rearrangement

Tetrahydropyran epoxides 12, the synthesis of which involves a RCM, undergo a base-induced rearrangement to 3,4-dihydro-2//-pyran-4-ols 13. These compounds are converted stereospecifically to 3,6-dihydropyrans 14 on treatment with allyltrimethylsilane (Scheme 6) <00EJO3145>. 

Transformation of the 7-oxo-2-enimides 28, available from chiral syn-aldols by a Cope rearrangement, into enantiopure tetrahydropyrans involves reduction of the aldehyde function followed by a fast intramolecular oxa Michael addition. The stereochemical course of the... 

One potentially interesting aspect of this particular system, and Claisen rearrangements in general, is the close structural resemblance of the transition state to the tetrahydropyran ring. 

Recall that in the latter, certain types of substituents adjacent to oxygen in the ring actually prefer axial arrangements. This observation has been codified in what is commonly referred to as the anomeric effect and is responsible in part for the conformations of carbohydrates. Is it possible that conformational preferences seen in substituted tetrahydropyrans will carry over into preferences in transition-state geometries for Claisen rearrangements ... 

Similarly, Lewis acid-catalyzed rearrangement of epoxy alcohols 84 (Scheme 37) gives in most cases the exo-cyclized tetrahydropyrans 85 as the main products (80-97%) however, with La(OTf)3 as the catalyst, the main products (80-90%) are endo-cyclized oxepanes 86 <1998TL393>. 

The proposed mechanism for the reaction is shown in Scheme 13.75. In the first step, the oxonium cation 208, formed by TfOH-catalyzed condensation of an aldehyde with alcohol 206, undergoes an intramolecular cyclization to form the tertiary carbocation 209. In a subsequent step, cation 209 undergoes a pinacol rearrangement, leading to the observed tetrahydropyran 205. 

Let s now consider concerted skeletal rearrangements which allow the direct transformation of a sugar structure into a carbocyde. The Claisen rearrangement has been used for the direct conversion of sugar C-glycosides to eight membered carbocycles, respectively. The reaction has precedents in the transformation of 2-methylene-6-vinyl-tetrahydropyrans to cyclooctenone derivatives, a transformation that has been applied in the synthesis of several natural products [54],... 

Rearrangement of the epoxy ester 317 affords a 1 1 mixture of the tetrahydropyran 318 and a steroidal tetra-hydrofuran by-product. This methodology offers a new hypothesis regarding the biosynthesis of polyether natural... 

A diastereoselective synthesis of tetrahydropyran-3-ones 1021 can be achieved via [2,3]-sigmatropic rearrangement of the ylides 1022, generated from the achiral diazoketone 1023 via formation of a copper carbenoid followed by an... 

The acid-catalysed rearrangement of 2,4,5-triols leads to the thermodynamic product, a tetrahydrofuran. However, cyclisation using trimethyl orthoacetate and pyridinium toluene-p-sulfonate gives a mixture of the THF and a tetrahydropyran, in which the former predominates, by attack of the primary hydroxy group at different ends of the episulfonium ion intermediate. This route nevertheless provides a useful route to THPs through equilibration of this reaction mixture <02JCS(P1)2646,02JCS(P1)2652>. 

Cyclobutenes possessing an angular O-functionality, obtained from a Lewis acid-mediated [2+2] cycloaddition of cyclic silyl enol ethers to ethyl propynoate and subsequent reduction and butenylation, undergo a ring-opening metathesis that produces a substituted dihydropyran that forms part of a c -diene. After desilylation, an oxy-Cope rearrangement leads to the fused tetrahydropyran 4 <03JA14901>. 

A sugar derivative related to the purine nucleosides, namely methyl 5-(adenin-l-yl)-5-deoxy-2,3-0-isopropylidene-j8-D-ribofurano-side, is formed by the reaction of methyl 5-amino-5-deoxy-2,3-0-iso-propylidene-jS-D-ribofuranoside with l-benzyl-5-cyano-4[(ethoxy-methylene)amino]imidazole, followed by removal of the benzyl group. On fusion, this compound rearranges to methyl 5-deoxy-2,3-0-isopropylidene-5-[(purin-6-yl)amino]-/8-D-ribofuranoside, which can also be obtained from the reaction of methyl 5-amino-5-deoxy-2,3-0-isopropylidene-/8-D-ribofuranoside with 6-chloro-9-(tetrahydropyran-2-yl)purine. In the same manner, methyl 5-amino-5-deoxy-2,3-di-0-p-tolylsulfonyl-/3-D-ribofuranoside reacts with 6-chloro-9-(tetrahydro-pyran-2-yl)purine to give methyl 5-deoxy-5-[9-(tetrahydropyran-2-yl)-purin-6-yl]amino-2,3-di-0-p-tolylsulfonyl-j8-D-ribofuranoside. With liquid ammonia, 5 -0-p-tolylsulfonyladenosine gives 5 -amino-5 -deoxyadenosine. On heating in p-dioxane, ring closure to 3,5 -an-hydroadenosine is observed. ... 

---